There is significant commercial interest in the fabrication of customized glass laminates with encapsulated digitally printed images having vibrant colors for use in architectural and automotive applications. Because project delivery times for current methods of producing limited quantities of glass laminates with customized graphics can be very long with accompanying high costs, there is a strong market need for methods that will allow rapid turnaround time at reasonable cost.
Up to now options to create such laminates have been virtually limited to traditional screen printing methods involving the deposition of solvent based colored inks onto plastic substrates and subsequent encapsulation of the printed film in glass using multiple layers of adhesive. This screen printing process involves time consuming and costly preparation of multiple screensxe2x80x94one for each color separation. Issues related to the use of solvents must be managed in order to prevent environmental problems. In addition, the required setup and cleanup times result in a process that is not cost effective for limited quantities of printed film.
Introduction of digital methods such as inkjet printing seems like a natural fit for production of glass laminates with customized graphics. However, inkjet printing onto clear substrates suffers from the lack of visual quality due to the less than vibrant colors that are obtained. Most commercial uses of inkjet printing utilize opaque substrates such as white paper or white polyvinyl chloride film (white vinyl) for optimum appearance and when such printing is performed on clear print media such as polyester film, there is a significant loss in color vibrancy. Because of the high ink pigment loading and large pigment particle size that would be required to print more opaque, and hence more vibrant colors onto a clear film, it is not likely that this color issue will be easily resolved for ink jet printing. Inkjet printing onto high opacity media such as white polyester or vinyl might result in acceptable appearance with vibrant colors but the resulting glass laminate would have poor see-through characteristics and lose much of the desired aesthetic value.
Encapsulation of thermal transfer printed color images in glass laminates for customized applications provides the opportunity to deliver the desired laminate appearance. Thermal transfer color printing was developed in early 1980""s and first used in commercial color printers for corporate office printing. In the mid-1990""s inkjet printing technology became dominant because of its much lower cost. Thermal transfer printing is still broadly used today for numerous applications such as printing bar codes onto labels and tags.
Thermal transfer printing is a dry-imaging process that involves the use of a printhead containing many resistive heating elements that selectively transfer solid ink from a coated ribbon to a substrate. As the coated ribbon is transported through the print head, targeted areas of the ink layer are heated, softened and transferred to the substrate. The consumed ribbon is usually rewound and disposed.
The resolution of a typical thermal transfer printer is usually around 200-400 dpi with software capability to utilize variable dot shapes and screen angles so that output quality can be very high quality depending upon media used. Because the ink is not required to pass through a small nozzle in the printhead, larger pigment particles and greater pigment loadings can be used with thermal transfer printing to achieve the desired vibrancy of color. However, achieving acceptable quality on clear media is more challenging than opaque media and because there is little commercial activity in this area, the print media choices are limited which can also affect the quality.
One of the major advantages of thermal transfer printing is the minimal setup times required to produce an image which reduces the cost and turnaround time of a short run as compared to traditional screen printing operations.
There are numerous types of ink formulations used for thermal transfer print ribbons including those that are primarily wax, wax/resin or resin based. Resin based ribbons are usually more expensive and are primarily used for production of more durable images with the ability to withstand outdoor exposure for up to 3-5 years without lamination. Wax based ribbons are usually less expensive and used for less demanding applications.
Thermal transfer printing has been used for many years in the printing of bar codes on labels, tags, and tickets and the technology for production of these ribbons has become very specialized.
A typical color ribbon is a relatively complex composite structure that has been developed to provide for optimum performance in the thermal transfer printing process. A typical high performance thermal transfer color ribbon consists of a very thin biaxially oriented polyethylene terephthalate (PET) film substrate usually with a thickness of xcx9c3-6 micrometers that acts as a carrier or support layer for the ink layer(s). PET film is selected as the preferred substrate because of its physical properties and ability to withstand print head temperatures of up to 120xc2x0 C. This PET substrate is coated on one side with at least one thin layer of pigmented resin. With many resin based color ribbons there is also a release layer between the PET substrate and the pigmented ink layer to facilitate transfer of the ink layer to the print media. Such a release layer will end up on top of the image that in many applications will provide additional protection for the printed image. On the other side of the thin substrate is usually a xe2x80x9cbackcoatingxe2x80x9d that provides the correct frictional properties between the printhead and the ribbon.
U.S. Pat. No. 5,939,207, the contents of which are incorporated herein by reference, describes the composition of a four layer thermal transfer ribbon structure for use in the printing of black bar codes. With minor changes, this structure is thought to be representative of a typical color ribbon formulation utilized with the present invention. This structure described in the ""207 Patent comprises a heat-resistant backcoat bonded to one side of a thermally and dimensionally stable substrate, such as PET film. An ultra thin release layer is provided on the other side of the substrate, with a pigmented layer then being provided on the release layer.
The pigmented layer contains carbon black and resin binder including polystryrene and polyacrylate resin with various functional groups such as methacrylic acid to promote adhesion to a variety of printing substrates. During printing, the pigmented layer is transferred to the print medium. Formulations for color ribbons used in applications requiring exterior durability will likely utilize a resin binder containing only polyacrylate resins and colored pigments with superior UV light stability. The formulation of the pigmented layer may also contain various waxes and other additives in order to achieve targeted viscosity and physical properties for optimum printing and coating performance.
In its final printed form, it is the release layer which if present functions as the top surface of the printed image. As described in the ""207 Patent, the formulation of this layer contains components that provide for easy release of the pigmented layer from the substrate and may include such components as ethylene vinyl acetate copolymer, anxe2x80x94olefin maleic anhydride copolymer and various waxes such as Carnauba wax.
Thermal transfer printing offers various color options including the standard cyan, magenta, yellow and black (C-M-Y-K) process colors as well as a wide range of spot colors including white, metallics, fluorescents and specialized colors. The ability to print process colors onto either a clear substrate or on a white printed background provides the opportunity to generate a unique combination of vibrant colors and see-through laminate appearance that is not possible with inkjet printing.
Thermal transfer printing can be used to print images on a wide variety of substrates including PET, paper, vinyl, etc. Accurate color transfer during printing, ink/substrate adhesion and overall visual appearance are usually dependent upon the surface characteristics, such as surface smoothness of the print media. There are a number of companies, including Dunmore Corporation of Bristol, Pa. who supply print media for use with thermal transfer printing. Ribbon suppliers such as IIMAK/T2 Solutions of Amherst, N.Y. will usually provide lists of qualified media to customers. In many cases there is a proprietary or patented coating on the print surface of the substrate that provides for improved ink transfer characteristics. Warranty of outdoor color durability of the printed image by the ribbon supplier is usually dependent upon use of qualified media.
Recent advances in thermal printing technology include the introduction of lower cost and higher performance color ribbons with excellent outdoor durability. IIMAK of Amherst, N.Y. makes a line of high performance resin based color ribbons called DuraCoat(copyright) (DC-300 series).
For greater productivity, thermal transfer printing equipment, such as the Sprinter B Printer manufactured by Matan, Ltd./Israel, are designed with multiple print heads and ribbons. Use of multiple print heads and corresponding ribbons require the use of a substrate with high enough modulus so that color registration or misaligned color placement is not an issue. For example, qualified thermal transfer printing media such as white vinyl needs to be reinforced with a xe2x80x9cbackingxe2x80x9d to minimize this issue. Commercial printers with width capabilities up to 132 cm are available.
In contrast to the traditional screen print approach for creation of vibrant colored images, the use of thermal transfer printing provides a very quick means to finished printed images. Use of digital image manipulation software such as Adobe""s Photoshop(copyright) and/or Illustrator(copyright) in combination with the thermal transfer printer""s raster image processing (RIP) software can complete a full project from design to finished proof in a matter of hours as compared to days or longer for the screen print process. This combination of thermal transfer printing and commercially available graphics software provides great opportunity for a wide range of customized aesthetics for use in laminated glass.
In glass laminate applications involving a glass/plasticized polyvinyl butyral (plasticized PVB)/glass combination, direct printing onto the plasticized PVB surface is not a viable option. The poor dimensional stability of plasticized PVB causes significant problems with color registration and poor appearance. Surface texture normally present on commercially available plasticized PVB interlayers for facilitating effective glass laminate processing also presents a significant problem in achieving acceptable appearance of the printed image. For these reasons accurate transfer of multiple colors onto plasticized PVB substrates has previously been found to be virtually impossible. It would thus be desirable to develop a laminate composite in which digital color images are printed onto a sufficiently rigid and smooth substrate which is then incorporated into the glass/plasticized PVB/glass laminate.
Dunmore Corporation produces a clear polyethylene terephthalate (PET) called Dun-Kote(copyright) DP38 film coated with its own proprietary coating specifically designed for thermal transfer printing. However, printing directly onto this substrate and subsequently incorporating the printed film into a glass laminate via encapsulation with two layers of plasticized PVB has been found to yield a laminate with inadequate structural integrity and with inconsistent printed image quality. These problems have been found to be the result of poor adhesion between the plasticized PVB and printed film componentxe2x80x94both on the printed and unprinted surfaces as well as unacceptable interactions between the PVB plasticizer and the printed side of the PET film. It would thus be desirable to develop a laminate composite that provides a means for incorporation of digitally printed color images into glass/plasticized PVB/glass laminates for customized applications.
The intent of this invention is to achieve a glass laminate with an encapsulated digitally printed image that has overall performance characteristics including penetration resistance, optical quality and durability equivalent to a conventional glass/plasticized PVB/glass laminate. Because of the well recognized barrier characteristics of PET film, use of a second PET film component that is laminated to the printed PET film avoids any interaction between the PVB plasticizer and the print media coating or ink layers.
This invention is directed to a plastic composite comprising color images printed onto a polyethylene terephthalate (PET) substrate using thermal transfer printing technology. The PET layer is then bonded to a second PET layer using a thermally activated adhesive, preferably ethylene vinyl acetate copolymer (EVA). The bonded PET layers are then disposed between two layers of plasticized PVB, forming the plastic composite. The plastic composite can then be placed between two sheets of glass forming the final laminated glass product.